1. Field of the Invention
The invention in general relates to electrosurgery and in particular to an electrosurgical unit utilizing solid state electronics.
2. Description of the Prior Art
Electrosurgical generators which produce high frequency electric currents used for cutting of tissue and coagulation of small blood vessels have been well-known for at least fifty years. The use of solid state devices in the electronics, and in particular the amplifiers of such electrosurgical devices has also been well-known for many years.
The use of so-called cascode configuration in which one electrical component provides a current gain while another component provides a voltage gain has been known in the electronics field for at least a generation. In particular, a cascode configuration consisting of a bipolar transistor and a field effect transistor (FET) connected in series, called a hybrid cascode configuration, has been known for many years. See, for example, United Kingdom Patent Application GB No. 2 053 606 A published Feb. 4, 1981, and the cascode configuration shown on page 6 of the Modern Electronic Circuits Reference Manual by John Markus, (McGraw Hill 1980).
Although the cascode configuration of solid state amplification units has been known for some time, it has not been used in electrosurgical generator amplifiers. The most common amplifier configuration for elecrtrosurgical units is the common emitter topolgy employing multiple parallel bipolar transistors. The only known use of FETS in the electrosurgical amplifier art has been employed in a full-H-bridge-type circuit. See United Kingdom Patent Application GB No. 2 105 200 A on an invention of William Joseph Bowers. In particular, a series-type configuration in which both of the semiconductors are used as amplifiers, rather than only as switches, has been believed to be not appropriate for electrosurgical units. This is because electrosurgical generators require fast switching at very high power, and significantly, they require the fast switching to be provided over a wide range of power levels. Such a use of the semiconductor device necessarily requires that the device be used in instances when it is not in saturation, in which cases the devices are highly inefficient. Those skilled in the art have believed that this would lead to high power dissipation in an electrosurgical unit with the resultant inefficiency, unreliability, and/or the necessity for ventilating fans which exhaust non-sterile air into the operating room environment.
It has for some time been known in the art that an electrosurgical generator that is capable of producing four different types of waveforms, including a cutting waveform, a coagulation waveform, a blend of cut and coagulation, and a fulguration waveform would be highly desirable, since surgeons commonly may require the use of all four types of functions in a given surgical procedure. In order to produce such combination type electrosurgical devices it has been necessary in the prior art to use highly complex amplification circuitry involved either separate amplification circuits for each waveform, complex switching arrangements between the circuit devices which in effect create different circuits for the various waveforms, or compromises in the performance and efficiency in one or more of the modes. See, for example, United Kingdom Patent Application GB No. 2 105 200 A referenced above, U.S. Pat. No. 4,188,927 issued to Frank W. Harris, and U.S. Pat. No. 3,952,748 issued to Paul L. Kaliher et al.